Transport Layer Security (TLS) Extension for Token Binding Protocol Negotiation
draft-ietf-tokbind-negotiation-13

Abstract

This document specifies a Transport Layer Security (TLS) extension for the negotiation of Token Binding protocol version and key parameters.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on November 10, 2018.

Copyright Notice

Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Requirements Language
• 2. Token Binding Negotiation Client Hello Extension
• 3. Token Binding Negotiation Server Hello Extension
• 4. Negotiating Token Binding Protocol Version and Key Parameters
• 5. IANA Considerations
• 6. Security Considerations
• 6.1. Downgrade Attacks
• 6.2. Triple Handshake Vulnerability in TLS 1.2 and Older TLS Versions
• 7. Acknowledgements
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Authors' Addresses

1. Introduction

In order to use the Token Binding protocol [I-D.ietf-tokbind-protocol], the client and server need to agree on the Token Binding protocol version and the parameters (signature algorithm, length) of the Token Binding key. This document specifies a new TLS [RFC5246] extension to accomplish this negotiation without introducing additional network round-trips in TLS 1.2 and earlier versions. The negotiation of the Token Binding protocol and key parameters in combination with TLS 1.3 and later versions is beyond the scope of this document.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

2. Token Binding Negotiation Client Hello Extension

The client uses the "token_binding" TLS extension to indicate the highest supported Token Binding protocol version and key parameters.

enum {
    token_binding(24), (65535)
} ExtensionType;
        

The "extension_data" field of this extension contains a "TokenBindingParameters" value.

struct {
    uint8 major;
    uint8 minor;
} TB_ProtocolVersion;

enum {
    rsa2048_pkcs1.5(0), rsa2048_pss(1), ecdsap256(2), (255)
} TokenBindingKeyParameters;

struct {
    TB_ProtocolVersion token_binding_version;
    TokenBindingKeyParameters key_parameters_list<1..2^8-1>
} TokenBindingParameters;
        

"token_binding_version" indicates the version of the Token Binding protocol the client wishes to use during this connection. If the client supports multiple Token Binding protocol versions, it SHOULD indicate the latest supported version (the one with the highest TB_ProtocolVersion.major and TB_ProtocolVersion.minor) in TokenBindingParameters.token_binding_version. E.g. if the client supports versions {1, 0} and {0, 13} of the Token Binding protocol, it SHOULD indicate version {1, 0}. Please note that the server MAY select any lower protocol version, see Section 3 "Token Binding Negotiation Server Hello Extension" for more details. If the client does not support the Token Binding protocol version selected by the server, then the connection proceeds without Token Binding. [I-D.ietf-tokbind-protocol] describes version {1, 0} of the protocol.

RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH: Prototype implementations of Token Binding drafts can indicate support of a specific draft version, e.g. {0, 1} or {0, 2}.

"key_parameters_list" contains the list of identifiers of the Token Binding key parameters supported by the client, in descending order of preference. [I-D.ietf-tokbind-protocol] establishes an IANA registry for Token Binding key parameter identifiers.

3. Token Binding Negotiation Server Hello Extension

The server uses the "token_binding" TLS extension to indicate support for the Token Binding protocol and to select the protocol version and key parameters.

The server that supports Token Binding and receives a client hello message containing the "token_binding" extension will include the "token_binding" extension in the server hello if all of the following conditions are satisfied:

1. The server supports the Token Binding protocol version offered by the client or a lower version.
2. The server finds acceptable Token Binding key parameters on the client's list.
3. The server is also negotiating the Extended Master Secret [RFC7627] and Renegotiation Indication [RFC5746] TLS extensions. This requirement applies when TLS 1.2 or an older TLS version is used (see Section 6 "Security Considerations" below for more details).

The server will ignore any key parameters that it does not recognize. The "extension_data" field of the "token_binding" extension is structured the same as described above for the client "extension_data".

"token_binding_version" contains the lower of:

• the Token Binding protocol version offered by the client in the "token_binding" extension and
• the highest Token Binding protocol version supported by the server.

"key_parameters_list" contains exactly one Token Binding key parameters identifier selected by the server from the client's list.

4. Negotiating Token Binding Protocol Version and Key Parameters

It is expected that a server will have a list of Token Binding key parameters identifiers that it supports, in preference order. The server MUST only select an identifier that the client offered. The server SHOULD select the most highly preferred key parameters identifier it supports which is also advertised by the client. In the event that the server supports none of the key parameters that the client advertises, then the server MUST NOT include "token_binding" extension in the server hello.

The client receiving the "token_binding" extension MUST terminate the handshake with a fatal "unsupported_extension" alert if any of the following conditions are true:

1. The client did not include the "token_binding" extension in the client hello.
2. "token_binding_version" is higher than the Token Binding protocol version advertised by the client.
3. "key_parameters_list" includes more than one Token Binding key parameters identifier.
4. "key_parameters_list" includes an identifier that was not advertised by the client.
5. TLS 1.2 or an older TLS version is used, but the Extended Master Secret [RFC7627] and TLS Renegotiation Indication [RFC5746] extensions are not negotiated (see Section 6 "Security Considerations" below for more details).

If the "token_binding" extension is included in the server hello and the client supports the Token Binding protocol version selected by the server, it means that the version and key parameters have been negotiated between the client and the server and SHALL be definitive for the TLS connection. TLS 1.2 and earlier versions support renegotiation, allowing the client and server to renegotiate the Token Binding protocol version and key parameters on the same connection. The client MUST use the negotiated key parameters in the "provided_token_binding" as described in [I-D.ietf-tokbind-protocol].

If the client does not support the Token Binding protocol version selected by the server, then the connection proceeds without Token Binding. There is no requirement for the client to support any Token Binding versions other than the one advertised in the client's "token_binding" extension.

Client and server applications can choose to handle failure to negotiate Token Binding in a variety of ways, e.g.: continue using the connection as usual, shorten the lifetime of tokens issued during this connection, require stronger authentication, terminate the connection, etc.

The Token Binding protocol version and key parameters are negotiated for each TLS connection, which means that the client and server include their "token_binding" extensions both in the full TLS handshake that establishes a new TLS session and in the subsequent abbreviated TLS handshakes that resume the TLS session.

5. IANA Considerations

This document updates the TLS "ExtensionType Values" registry. IANA has provided the following temporary registration for the "token_binding" TLS extension:

• Value: 24
• Extension name: token_binding
• Reference: this document
• Recommended: Yes

IANA is requested to make this registration permanent, keeping the value of 24, which has been used by the prototype implementations of the Token Binding protocol.

This document uses "Token Binding Key Parameters" registry originally created in [I-D.ietf-tokbind-protocol]. This document creates no new registrations in this registry.

6. Security Considerations

6.1. Downgrade Attacks

The Token Binding protocol version and key parameters are negotiated via "token_binding" extension within the TLS handshake. TLS prevents active attackers from modifying the messages of the TLS handshake, therefore it is not possible for the attacker to remove or modify the "token_binding" extension. The signature algorithm and key length used in the Token Binding of type "provided_token_binding" MUST match the parameters negotiated via "token_binding" extension.

6.2. Triple Handshake Vulnerability in TLS 1.2 and Older TLS Versions

The Token Binding protocol relies on the TLS Exporters [RFC5705] to associate a TLS connection with a Token Binding. The triple handshake attack [TRIPLE-HS] is a known vulnerability in TLS 1.2 and older TLS versions, allowing the attacker to synchronize keying material between TLS connections. The attacker can then successfully replay bound tokens. For this reason, the Token Binding protocol MUST NOT be negotiated with these TLS versions, unless the Extended Master Secret [RFC7627] and Renegotiation Indication [RFC5746] TLS extensions have also been negotiated.

7. Acknowledgements

This document incorporates comments and suggestions offered by Eric Rescorla, Gabriel Montenegro, Martin Thomson, Vinod Anupam, Anthony Nadalin, Michael B. Jones, Bill Cox, Nick Harper, Brian Campbell and others.

This document was produced under the chairmanship of John Bradley and Leif Johansson. The area directors included Eric Rescorla, Kathleen Moriarty and Stephen Farrell.

8. References

8.1. Normative References

8.2. Informative References

Authors' Addresses